U.S. patent number 7,116,399 [Application Number 10/844,570] was granted by the patent office on 2006-10-03 for lithographic apparatus, device manufacturing method, and device manufactured thereby.
This patent grant is currently assigned to ASML Netherlands B.V.. Invention is credited to Wilhelmus Josephus Box, Dominicus Jacobus Petrus Adrianus Franken, Martinus Hendrikus Antonius Leenders, Erik Roelof Loopstra, Josephus Jacobus Smits, Marc Wilhelmus Maria Van Der Wijst, Antonius Johannes Josephus Van Dijsseldonk.
United States Patent |
7,116,399 |
Box , et al. |
October 3, 2006 |
Lithographic apparatus, device manufacturing method, and device
manufactured thereby
Abstract
A lithographic projection apparatus contains a projection system
configured to project a patterned beam of radiation onto a target
portion of a substrate. The projection system contains one or more
optically active mirrors and heat shields located to intercept heat
radiation to or from the mirrors and/or their support. The heat
shields are actively cooled and the mirrors and the heat shields
and the mirrors are supported separately on a support frame to
reduce vibration of the mirrors due to active cooling. The heat
shields may include heat shields that intercept heat radiation to
or from the support and/or respective heat shields for individual
mirrors that intercept heat radiation to or from the mirrors.
Inventors: |
Box; Wilhelmus Josephus (Eksel,
BE), Van Dijsseldonk; Antonius Johannes Josephus
(Hapert, NL), Franken; Dominicus Jacobus Petrus
Adrianus (Veldhoven, NL), Leenders; Martinus
Hendrikus Antonius (Rotterdam, NL), Loopstra; Erik
Roelof (Heeze, NL), Smits; Josephus Jacobus
(Geldrop, NL), Van Der Wijst; Marc Wilhelmus Maria
(Veldhoven, NL) |
Assignee: |
ASML Netherlands B.V.
(Veldhoven, NL)
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Family
ID: |
33016938 |
Appl.
No.: |
10/844,570 |
Filed: |
May 13, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050018154 A1 |
Jan 27, 2005 |
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Foreign Application Priority Data
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May 13, 2003 [EP] |
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03076433 |
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Current U.S.
Class: |
355/53; 359/820;
355/67 |
Current CPC
Class: |
G03F
7/70808 (20130101); G03F 7/70858 (20130101); G03F
7/70891 (20130101); G03F 7/709 (20130101); G03F
7/70833 (20130101); G03F 7/70991 (20130101); G03F
7/70825 (20130101) |
Current International
Class: |
G03B
27/42 (20060101); G02B 7/02 (20060101); G03B
27/54 (20060101) |
Field of
Search: |
;355/30,53-67
;359/811-820 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2004-29314 |
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Jan 2001 |
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JP |
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2003-234276 |
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Aug 2003 |
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JP |
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2004-80025 |
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Mar 2004 |
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JP |
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Primary Examiner: Nguyen; Henry Hung
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman,
LLP
Claims
What is claimed is:
1. A lithographic projection apparatus, comprising: a radiation
system configured to provide a beam of radiation; a first support
configured to support a patterning device, the patterning device
configured to pattern the beam according to a desired pattern; a
substrate table configured to hold a substrate; and a projection
system configured to project the patterned beam onto a target
portion of the substrate, the projection system comprising: an
optically active mirror; a second support configured to support at
least the mirror; and a heat radiation shield located to intercept
heat radiation to or from surfaces of at least one of the second
support and the mirror; a heat transport circuit, in thermal
contact with the heat radiation shield, configured to transport
heat to or from the radiation shield; a third support including
respective separate support elements thereon to support the second
support and the heat radiation shield on the third support, the
support elements and the at least one heat shield being
mechanically free from one another except for the support of the
third support.
2. A lithographic projection apparatus according to claim 1,
wherein the heat radiation shield comprises an outer shield
adjacent to an outer surface of the second support that faces away
from the mirror.
3. A lithographic projection apparatus according to claim 2,
wherein the outer shield has a higher absorption coefficient for
heat radiation on a side facing the outer surface of the second
support than on a side facing away from the outer surface.
4. A lithographic projection apparatus according to claim 2,
wherein the mirror has mutually opposite sides along an axis and
the second support faces the mirror on either of the sides.
5. A lithographic projection apparatus according to claim 1,
wherein the mirror has a front surface from which the beam is
reflected and a back surface opposite the front surface, the heat
radiation shield comprising a mirror shield adjacent the back
surfaces of the mirror, leaving free at least a part of the front
surface on which the beam is incident.
6. A lithographic projection apparatus according to claim 5,
wherein the at least one mirror shield substantially surrounds the
mirror, except at at least one of a part of the front surface on
which the beam is incident, attachment points to the second
support, and interfaces for at least one of sensors and
actuators.
7. A lithographic projection apparatus according to claim 5,
wherein the mirror shield has a higher absorption coefficient for
heat radiation on a side facing the surfaces of the mirror than on
a side facing away from the surfaces.
8. A lithographic projection apparatus according to claim 1,
wherein the heat radiation shield comprises an inner shield
adjacent an inner surface of the second support that faces the
mirror.
9. A lithographic projection apparatus according to claim 8,
wherein the inner shield has a higher absorption coefficient for
heat radiation on a side facing the inner surface than on a side
facing away from the inner surface.
10. A lithographic projection apparatus according to claim 1,
further comprising: a heat transport regulation loop including a
temperature sensor coupled to at least one of the heat radiation
shield, the second support and the mirror; and a regulating output
coupled to the heat transfer circuit configured to regulate the
amount of heat transported by the heat transfer circuit so that a
set temperature is achieved in the at least one of the heat
radiation shield, the second support and the mirror.
11. A device manufacturing method, comprising: projecting a
patterned beam of radiation onto a target portion of a layer of
radiation-sensitive material on a substrate via an optically active
mirror; shielding heat radiation to or from at least one of the
optically active mirror and a support for the optically active
mirror with a heat radiation shield, wherein the mirror and the
support as a whole are supported separately from the heat radiation
shield, supplying heat transfer fluid to the heat radiation shield
through pipes that are mechanically attached to the heat radiation
shield.
12. A device manufacturing method according to claim 11, further
comprising: absorbing heat radiated from an outer surface of the
support that faces away from the mirror with an outer shield
adjacent the outer surface and included in the heat radiation
shield.
13. A device manufacturing method according to claim 11, further
comprising: absorbing heat radiated from a back surface of the
mirror that faces away from a front surface from which the beam is
reflected with a mirror shield included in the heat radiation
shield, adjacent the back surfaces of the mirror, leaving the front
surface free.
14. A device manufacturing method according to claim 11, further
comprising: absorbing heat radiated towards an outer surface of the
support that faces away from the mirror with an outer shield
adjacent the outer surface and included in the heat radiation
shield.
15. A device manufacturing method according to claim 11, further
comprising: absorbing heat radiated from an inner surface of the
support that faces the mirror with an inner shield included in the
heat radiation shield adjacent the inner surface of the
support.
16. A lithographic projection apparatus, comprising: a radiation
system configured to provide a beam of radiation; a first support
configured to support a patterning device, the patterning device
configured to pattern the beam according to a desired pattern; a
substrate table configured to hold a substrate; and a projection
system configured to project the patterned beam onto a target
portion of the substrate, the projection system comprising an
optically active mirror, a second support configured to support the
mirror and an outer shield adjacent to an outer surface of the
second support that faces away from the mirror and located to
intercept heat radiation to or from at least one of the outer
surface of the second support and the mirror.
Description
This application claims priority to European Patent Application
03076433.6, filed May 13, 2003, the contents of which are
incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a lithographic projection
apparatus, a device manufacturing method and a device manufactured
thereby.
DESCRIPTION OF THE RELATED ART
The term "patterning devices" as here employed should be broadly
interpreted as referring to a device that can be used to endow an
incoming radiation beam with a patterned cross-section,
corresponding to a pattern that is to be created in a target
portion of the substrate. The term "light valve" can also be used
in this context. Generally, the pattern will correspond to a
particular functional layer in a device being created in the target
portion, such as an integrated circuit or other device (see below).
An example of such a patterning devices is a mask. The concept of a
mask is well known in lithography, and it includes mask types such
as binary, alternating phase-shift, and attenuated phase-shift, as
well as various hybrid mask types. Placement of such a mask in the
radiation beam causes selective transmission (in the case of a
transmissive mask) or reflection (in the ease of a reflective mask)
of the radiation impinging on the mask, according to the pattern on
the mask. In the case of a mask, the support will generally be a
mask table, which ensures that the mask can be held at a desired
position in the incoming radiation beam, and that it can be moved
relative to the beam if so desired.
Another example of a patterning device is a programmable mirror
array. One example of such a device is a matrix-addressable surface
having a viscoelastic control layer and a reflective surface. The
basic principle behind such an apparatus is that, for example,
addressed areas of the reflective surface reflect incident light as
diffracted light, whereas unaddressed areas reflect incident light
as undiffracted light. Using an appropriate filter, the
undiffracted light can be filtered out of the reflected beam,
leaving only the diffracted light behind. In this manner, the beam
becomes patterned according to the addressing pattern of the
matrix-addressable surface. An alternative embodiment of a
programmable mirror array employs a matrix arrangement of tiny
mirrors, each of which can be individually tilted about an axis by
applying a suitable localized electric field, or by employing
piezoelectric actuators. Once again, the mirrors are
matrix-addressable, such that addressed mirrors will reflect an
incoming radiation beam in a different direction to unaddressed
mirrors. In this manner, the reflected beam is patterned according
to the addressing pattern of the matrix-addressable mirrors. The
matrix addressing can be performed using suitable electronics. In
both of the situations described hereinabove, the patterning
devices can include one or more programmable mirror arrays. More
information on mirror arrays as here referred to can be found in,
for example, U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT Patent
Application Publications WO 98/38597 and WO 98/33096, which are
incorporated herein by reference. In the case of a programmable
mirror array, the support may be embodied as a frame or table, for
example, which may be fixed or movable as needed.
Another example of a patterning device is a programmable LCD array.
An example of such a construction is given in U.S. Pat. No.
5,229,872, which is incorporated herein by reference. As above, the
support structure in this case may be embodied as a frame or table,
for example, which may be fixed or movable as needed.
For purposes of simplicity, the rest of this text may, at certain
locations, specifically direct itself to examples involving a mask
and mask table. However, the general principles discussed in such
instances should be seen in the broader context of the patterning
devices as set forth above.
Lithographic projection apparatus can be used, for example, in the
manufacture of integrated circuits (ICs). In such a case, the
patterning devices may generate a circuit pattern corresponding to
an individual layer of the IC, and this pattern can be imaged onto
a target portion (e.g. including one or more dies) on a substrate
(silicon wafer) that has been coated with a layer of
radiation-sensitive material (resist). In general, a single wafer
will contain a whole network of adjacent target portions that are
successively irradiated via the projection system, one at a time.
In current apparatus, employing patterning by a mask on a mask
table, a distinction can be made between two different types of
machine. In one type of lithographic projection apparatus, each
target portion is irradiated by exposing the entire mask pattern
onto the target portion at once. Such an apparatus is commonly
referred to as a wafer stepper or step-and-repeat apparatus. In an
alternative apparatus, commonly referred to as a step-and-scan
apparatus, each target portion is irradiated by progressively
scanning the mask pattern under the beam in a given reference
direction (the "scanning" direction) while synchronously scanning
the substrate table parallel or anti-parallel to this direction.
Since, in general, the projection system will have a magnification
factor M (generally <1), the speed V at which the substrate
table is scanned will be a factor M times that at which the mask
table is scanned. More information with regard to lithographic
devices as here described can be found in, for example, U.S. Pat.
No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection
apparatus, a pattern (e.g. in a mask) is imaged onto a substrate
that is at least partially covered by a layer of
radiation-sensitive material (resist). Prior to this imaging, the
substrate may undergo various procedures, such as priming, resist
coating and a soft bake. After exposure, the substrate may be
subjected to other procedures, such as a post-exposure bake (PEB),
development, a hard bake and measurement/inspection of the imaged
features. This array of procedures is used as a basis to pattern an
individual layer of a device, e.g. an IC. Such a patterned layer
may then undergo various processes such as etching,
ion-implantation (doping), metallization, oxidation,
chemo-mechanical polishing, etc., all intended to finish off an
individual layer. If several layers are needed, then the whole
procedure, or a variant thereof, will have to be repeated for each
new layer. Eventually, an array of devices will be present on the
substrate (wafer). These devices are then separated from one
another by a technique such as dicing or sawing, whence the
individual devices can be mounted on a carrier, connected to pins,
etc. Further information regarding such processes can be obtained,
for example, from the book "Microchip Fabrication: A Practical
Guide to Semiconductor Processing", Third Edition, by Peter van
Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4,
incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter
be referred to as the "lens." However, this term should be broadly
interpreted as encompassing various types of projection systems,
including refractive optics, reflective optics, and catadioptric
systems, for example. The radiation system may also include
components operating according to any of these design types to
direct, shape and/or control the projection beam of radiation, and
such components may also be referred to below, collectively or
singularly, as a "lens." Further, the lithographic apparatus may be
of a type having two or more substrate tables (and/or two or more
mask tables). In such "multiple stage" devices the additional
tables may be used in parallel, or preparatory steps may be carried
out on one or more tables while one or more other tables are being
used for exposures. Dual stage lithographic apparatus are
described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796,
both incorporated herein by reference.
From European Patent Application Publication 1 178 357 ("EP '357"),
a lithographic apparatus is known of which most of the components
are located in a vacuum chamber. The beam of radiation images the
mask onto the substrate via a number of optically active mirrors
instead of lenses. Such a configuration is needed, for example,
when an EUV (extreme ultraviolet) beam is used because an EUV beam
in gases at atmospheric pressure would be useless for projection
purposes and because no refractive optical elements are available
for EUV radiation. The same holds for other types of beams.
EP '357 notes that operation under vacuum can cause temperature
stability problems, because heat radiation from the walls of the
vacuum chamber or from the vacuum pump could lead to thermal
expansion or contraction. This results in imaging errors when
temperature sensitive components, for example the support, the
substrate table, the projection system or the reference frame, are
affected. EP '357 addresses this problem with the use of a
"temperature control member," which is effectively a heat shield
interposed between heat sources and temperature sensitive
components. The heat shield surrounds at least part of the
temperature sensitive component that is kept isothermal. In one
embodiment the heat shield has a high absorption finish on a side
facing the temperature critical component, to regulate the
temperature of the temperature sensitive component by radiation
incident from the heat shield. By "regulating" EP '357 appears to
mean merely that radiation from the shield stabilizes the
temperature of the temperature sensitive component. No active
temperature control loop is shown.
Of course, this type of solution only works if the heat shield
itself does not heat to an arbitrary temperature. EP '357 does not
mention how this should be ensured in the presence of strong heat
sources. Moreover, absorption of radiation from the beam of
radiation in the projection system may turn the projection system
itself into both a temperature sensitive component and a heat
source at the same time, making it impossible to interpose heat
shields between all heat sources and temperature sensitive
components.
SUMMARY OF THE INVENTION
In principle, active heat transport, for example fluid cooling or
heating, may be used to control the temperature of temperature
sensitive components. It is also desirable that such active heat
transport be regulated by a control loop to keep temperatures
stable. However, this type of heat transport, when applied to the
projection system of a lithographic projection apparatus, may lead
to mechanical vibrations that result in imaging errors, for example
due to forced cooling or to the necessary mechanical connections.
Such problems increase when more cooling capacity is needed.
It is an aspect of the present invention to provide temperature
stability of the projection system of a lithographic apparatus with
active heat transport while minimizing the effect of mechanical
vibration due to heat transport.
It is another aspect of the present invention to make it possible
to use a control loop to regulate temperatures affecting the
projection system.
It is another aspect of the present invention to reduce heating
problems due to absorption of the beam in the projection system of
a lithographic apparatus.
According to an embodiment of the invention, mirrors of the
projection system and/or their support structure are shielded by
heat shields. An active heat transport circuit is used to thermally
condition the heat shields. The heat transport circuit may include
pipes to transport heat transport fluid coupled directly or
indirectly to the heat shields. Generally the heat transport has a
net cooling effect on the heat shields, but alternatively the heat
transport may have a net heating effect that is reduced if
temperature rises too much.
The heat shields are supported separately from the mirrors and
their support. As a result, vibrations due to active cooling have a
minimal effect on the mirrors. The only common support of the heat
shields and the mirrors and their support is a common metrology
frame, or on another frame that supports the mirrors via the
metrology frame. In one embodiment, both the heat shields and the
support are supported by the metrology frame and there is no other
mechanical interconnections between the heat shields and the
support or the mirrors.
The heat shields intercept heat radiation to or from the mirrors
and/or the support. The heat shields may a shape and be positioned
so that they intercept the major part (at least 50% and desirably
more than 80%) of heat radiation to or from the shielded support
and so that only a minor part of the intercepted heat radiation is
not to or from the shielded support or mirror. Respective heat
shields may each shield only a respective mirror or part of the
support as much as possible from all other components in this
way.
Thus, a minimum heat transport is needed for each heat shield,
reducing the amount of vibration and selective temperature
regulation of the mirrors and/or the support from the heat shields
is simplified. This may be achieved, for example, when the heat
shield tightly follows a surface of the relevant mirror or part of
the support, so that the distance between the heat shield and the
surface is smaller than the spatial extent of the heat shield.
In one embodiment the heat shield is provided for an outer surface
of the support of the mirrors that faces away from the mirrors.
Such an outer surface is most susceptible to heat radiated from
other components. The support may encase the mirrors except for
holes for passing the beam and/or for allowing a high vacuum to be
realized inside the support. More particularly the support of the
mirrors may be arranged so that it surrounds the mirrors on both
sides along at least one axis, heat shields being provided on the
outer surfaces on both sides.
The absorption coefficient of the surface of the heat shield that
faces away from the support may be lower than that of the surface
of the heat shield that faces the support structure (e.g. more than
0.8 on the surface facing the support and less than 0.2 on the
surface facing away from the support). Thus, the heat shield serves
to determine the temperature of the support and a minimum of
cooling is needed to compensate for heat absorbed from outside the
support. Alternatively a high absorption coefficient (>0.8) or a
low absorption coefficient (>0.2) may be used on both surfaces
at least locally, for example high absorption where there is
sufficient active cooling capacity on the surface and low
absorption where there is less cooling capacity.
In another embodiment the heat shields include mirror shields for
individual ones of the optically active mirrors of the projection
system. The mirror shield of a mirror does not cover a part of the
front surface of the mirror, from which the beam is reflected, but
otherwise the mirror heat shield may cover as much of the mirror as
possible, and at least a back surface opposite the front surface.
At least the mirror shield is located so that the major part of the
heat radiation that it intercepts is to or from the mirror that is
shielded, and the major part of the heat radiation to or from the
mirror except for heat radiation to or from the part of the front
surface from which the projection beam is reflected.
The mirror shield absorbs heat radiation generated by heating due
to beam reflection losses. By using mirror shields for individual
mirrors the amount of cooling is minimized as well as effects on
the support of the mirrors. The mirror shield may face at least a
back surface of the mirror, but may also be shaped so that it also
faces sides of the mirror between the front and the back surface of
the mirror, and even may be shaped to face a part of the front
surface on which the projection beam is not incident.
The surface of the mirror heat shield that faces the mirror may
have a higher absorption coefficient than the surface that faces
away from the mirror. Thus, heating of the mirror shield is
reduced, which may be desirable if, due to space limitations, only
small heat transfer capacity is available for a mirror.
Alternatively a high absorption coefficient (>0.8) may be used
on the surface of the mirror shield facing away from the mirror, to
prevent reflection of heat radiation towards other mirrors or the
support structure.
In another embodiment the heat shields include an inner heat shield
that covers an inner surface of the support, which faces one or
more of the mirrors. The surface of the inner heat shield that
faces the mirrors may have a lower absorption coefficient than the
surface that faces away from the mirror to the support structure.
Thus, heating of the mirrors is reduced. Alternatively a high
absorption coefficient (>0.8) may be used on the surface of the
mirror shield facing the mirrors, to prevent reflection of heat
radiation towards other mirrors or the support. Also a low
absorption coefficient may be used at least locally towards the
support.
The amount of heat transported by the heat transport circuit may be
regulated with a control loop that works to keep temperatures, such
as the heat shield temperature, substantially constant. When mirror
heat shields are provided for individual mirrors the temperature of
the mirrors is thus specifically controlled, with little or no
external influence. Similarly, the temperature of the support of
the mirrors is specifically controlled by providing inner and/or
outer shields. As a result, the optical properties of the
projection system remain stable.
According to a further aspect of the invention there is provided a
device manufacturing method in which heat radiation to or from the
one or more optically active mirrors and/or a support of one or
more optically active mirrors is shielded with one or more heat
shields that are supported separate from the one or more mirrors
and its support as a whole. Heat transfer fluid is supplied to the
one or more heat shields through pipes that are mechanically
attached to the heat shields.
Although specific reference may be made in this text to the use of
the apparatus according to the invention in the manufacture of ICs,
it should be explicitly understood that such an apparatus has many
other possible applications. For example, it may be employed in the
manufacture of integrated optical systems, guidance and detection
patterns for magnetic domain memories, liquid-crystal display
panels, thin-film magnetic heads, etc. It should be appreciated
that, in the context of such alternative applications, any use of
the terms "reticle", "wafer" or "die" in this text should be
considered as being replaced by the more general terms "mask",
"substrate" and "target portion", respectively.
In the present document, the terms "radiation" and "beam" are used
to encompass all types of electromagnetic radiation, including
ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248,
193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g.
having a wavelength in the range 5 20 nm), as well as particle
beams, such as ion beams or electron beams.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying schematic drawings
in which corresponding reference symbols indicate corresponding
parts, and in which:
FIG. 1 depicts a lithographic projection apparatus according to an
embodiment of the invention;
FIG. 2 shows a projection system;
FIG. 2a shows a detail of a projection system;
FIG. 3 shows a control loop;
FIG. 4 shows a number of alternative mirror shield configurations;
and
FIG. 5 shows a further mirror shield configuration.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a lithographic projection apparatus 1
according to an embodiment of the invention and including a
radiation system LA, IL configured to supply a beam PB of radiation
(e.g. EUV radiation). In this emobidment, the radiation system also
includes a radiation source LA; a first object table (mask table)
MT provided with a mask holder configured to hold a patterning
device, illustrated in the form of a mask MA (e.g. a reticle), and
connected to a first positioning device PM that accurately
positions the mask with respect to a projection system ("lens") PL.
A second object table (substrate table) WT is provided with a
substrate holder is configured to hold a substrate W (e.g. a
resist-coated silicon wafer) and is connected to a second
positioning device PW that accurately positions the substrate with
respect to the projection system PL. The projection system ("lens")
PL (e.g. a set of mirrors) is configured to image an irradiated
portion of the mask MA onto a target portion C (e.g. including one
or more dies) of the substrate W.
As here depicted, the apparatus is of a reflective type (i.e. has a
reflective mask). However, it may also be of a transmissive type,
for example (with a transmissive mask). Alternatively, the
apparatus may employ another kind of patterning device, such as a
programmable mirror array of a type as referred to above.
The source LA (e.g. a laser produced plasma source or a discharge
source) produces radiation. This radiation is fed into the
illumination system (illuminator) IL, either directly or after
having traversed a conditioning device(s), for example a beam
expander. The illuminator IL may includes components configured to
set the outer and/or inner radial extent (commonly referred to as
.sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in the beam. In addition, it will generally include
various other components, such as an integrator and a condenser. In
this way, the beam PB impinging on the mask MA has a desired
uniformity and intensity distribution in its cross-section.
It should be noted with regard to FIG. 1 that the source LA may be
within the housing of the lithographic projection apparatus (as is
often the case when the source LA is a mercury lamp, for example),
but that it may also be remote from the lithographic projection
apparatus, the radiation beam which it produces being led into the
apparatus (e.g. with the aid of suitable directing mirrors). The
present invention encompasses both of these scenarios.
The beam PB subsequently intercepts the mask MA, which is held on a
mask table MT. Having been selectively reflected by the mask MA,
the beam PB is processed by the projection system PL, which focuses
the beam PB onto a target portion C of the substrate W. With the
aid of the second positioning device PW (and interferometer IF),
the substrate table WT can be moved accurately, e.g. so as to
position different target portions C in the path of the beam PB.
Similarly, the first positioning device PM can be used to
accurately position the mask MA with respect to the path of the
beam PB, e.g. after mechanical retrieval of the mask MA from a mask
library, or during a scan. In general, movement of the object
tables MT, WT will be realized with the aid of a long-stroke module
(coarse positioning) and a short-stroke module (fine positioning),
which are not explicitly depicted in FIG. 1. However, in the case
of a wafer stepper (as opposed to a step-and-scan apparatus) the
mask table MT may just be connected to a short stroke actuator, or
may be fixed. Mask MA and substrate W may be aligned using mask
alignment marks M1, M2 and substrate alignment marks P1, P2.
The depicted apparatus can be used in two different modes: 1. In
step mode, the mask table MT is kept essentially stationary, and an
entire mask image is projected at once (i.e. a single "flash") onto
a target portion C. The substrate table WT is then shifted in the X
and/or Y directions so that a different target portion C can be
irradiated by the beam PB; and 2. In scan mode, essentially the
same scenario applies, except that a given target portion C is not
exposed in a single "flash". Instead, the mask table MT is movable
in a given direction (the so-called "scan direction", e.g. the Y
direction) with a speed v, so that the beam PB is caused to scan
over a mask image. Concurrently, the substrate table WT is
simultaneously moved in the same or opposite direction at a speed
V=Mv, in which M is the magnification of the lens PL (typically,
M=1/4 or 1/5). In this manner, a relatively large target portion C
can be exposed, without having to compromise on resolution.
The lithographic projection apparatus includes a vacuum chamber VC
in which the beam PB impinges on mask MA and subsequently onto the
target area of the substrate W. A metrology frame MF, which is
mechanically isolated from the main apparatus structure, provides
an isolated frame of reference. The metrology frame may be, for
example, a heavy table supported by air mounts (not shown) that
provide a resilient support with a low elastic coefficient. The
metrology frame MF supports sensitive components, for example the
interferometer IF and other position sensors and isolate them from
vibration.
The projection system PL is supported by the metrology frame ME via
a resilient element 12 and a support 11. The projection system PL
is partially surrounded by a shield 10, which includes its own
support 14 on metrology frame ME, and is coupled to a supply pipe
16 for heat transport fluid and an abduction pipe 18 for the heat
transport liquid.
FIG. 2 shows an embodiment ofprojection sytem PL and its shield
structure in more detail. The projection system PL contains a
number of optically active mirrors 20 (only one of which is
provided with a reference number) and a projection system support
26. The mirrors 20 are arranged to image the mask MA onto the
substrate W. The mirrors 20 and the support 26 are made of, for
example, a glass with low expansion coefficient such as
ZERODUR.RTM. or ULE.RTM. (Ultra Low Expansion) glass. INVAR.RTM.
may also be used for the support 26.
The support 26 may include legs to which the mirrors 20 are
attached and a box in which the mirrors 20 are placed and which
mostly surrounds the mirrors 20, substantially except for openings
through which the beam PB passes. Alternatively a cage may be used.
The box or cage with the mirrors 20 is supported on the metrology
frame MF.
The shield contains mirror shields 22 (only one of which is
provided with a reference number), an outer shield 28 and an inner
shield 29. As can be appreciated from FIG. 1, the optically active
mirrors 20, the support 26, the mirror shields 22, the outer shield
28 and the inner shield 29 each have their own support on metrology
frame MF. The mirror shields 22, the outer shield 28 and the inner
shield 29 may have a combined support on the metrology frame MF, or
separate supports may be used. The supports, particularly for the
shields, may alternatively be connected to the vacuum chamber VC or
to a base frame (not shown) other than the metrology frame. The
support 14 is supported via the resilient element 12 to isolate the
support 14 from vibrations of the metrology frame MF. Although only
one resilient element 12 is shoxkrn for the sake of clarity, it
should be appreciated that the support 14 may be supported via a
number of such elements in parallel. Similarly, the shields 28, 29,
22 may be supported on more than one point on metrology frame MF,
none of them via the resilient element 12, or other resilient
elements that support the projection system support 26.
FIG. 2a shows a detail of the projection system, illustrating the
mechanical connections 200 between support structure 26 and a
mirror 20, separated from mirror shield 22. Preferably, mirror
shield 22 has support connections 202 to inner shield 29, but
alternatively mirror shield 22 may be supported separately.
A respective mirror shield 22 is provided for each of mirrors 20.
Each mirror shield 22 tightly surrounds its mirror 20 substantially
entirely except for the face of mirror 20 that reflects the beam PB
and connections that support the mirror 20 on the support 26. The
exact geometrical arrangement of the mirror shields 22 may be
varied, but they are desirably shaped so that the majority of heat
radiation intercepted by the mirror shields 22 is heat radiation to
or from the mirror 20 and not between other structures. The mirror
shields 22 are arranged so that they intercept a major part
(e.g.>50%) of the heat radiation to or from an optically active
mirror 20, except for heat radiation from the part of the optically
active mirror 20 that reflects the beam PB.
FIG. 4 shows various configurations of a mirror shield 22, provided
only on the back surface 40 of an optically active mirror 20, or
following the shape of the optically active mirror 20 so that sides
42 are also covered, or even a part of a front surface 44 that is
not irradiated by the beam PB. The latter construction prevents
heating by other structures and heating of other structures as much
as possible.
Heat transport pipes 24 may be attached to the mirror shields 22.
The outer shield 28 is provided adjacent the face of support 26
that faces away from the mirrors 20 and the inner shield 29 is
provided adjacent the face of the support 26 that faces towards the
mirrors 20. Heat transport pipes 27 may be provided on both the
outer shield 28 and the inner shield 29. The heat transport pipes
24, 27 may be arranged in a serial circuit, or a parallel
circuit.
FIG. 5 shows an alternative embodiment, wherein a heat shield 52
extends along a direction along an average normal to the surface of
mirror 50. In this way, at least some of the radiation from the
reflecting surface of mirror 50 is intercepted by the cooled heat
shield 52. Preferably the heat shield 52 extends as far as possible
without intercepting the beam PB, typically at least one half of a
diameter of the mirror 50. The shield 52 may be cylindrical around
the normal, only the cross-section of the cylinder with a plane
though the mirror being shown in FIG. 5. However, parallel plane
shields may be used.
In operation the beam PB is incident on the mask MA. The mask MA is
imaged onto the substrate W by the optically active mirrors 20.
Heat transport fluid is fed through the heat transport pipes 24,
27. Water may be used as heat transport fluid. The mirror shields
22 limit the temperature swing of the mirrors 20. When the beam PB
is reflected by the mirrors 20 inevitably some absorption occurs.
The absorption may lead to imaging problems, when it affects the
geometrical relation between the mirrors 20, the mask MA and the
substrate W. When submicron accuracy is needed, very small
disturbances can already be damaging. This is especially a problem
for short wavelength beams PB, such as EUV beams, because such a
beam requires high vacuum (making it difficult to sink absorbed
heat) and involves relatively high absorption.
The mirror shields 22 absorb heat radiated from the mirrors 20.
Excess heat is removed via the heat transport pipes 24. Thus, the
effect of heating of the mirrors 20 is reduced and the temperature
of the mirrors 20 approximates a thermal equilibrium with the
mirror shields 22, as determined by the heat transport fluid. By
placing the mirror shields 22 tightly around the mirrors 20 it is
ensured that with a minimum of shield material a maximum of heat is
absorbed and as little as possible heat radiation from other
sources reaches the mirrors 20. Thus, a small heat transport
capacity suffices for the mirror shields 22. Because the mirror
shields 22 are supported on the metrology frame MF separately from
the mirrors 20, mechanical vibrations due to the flow of heat
transport fluid do not significantly influence the position of the
mirrors 20.
In addition, the mirror shields 22 prevent heat radiating from
other structures from heating the mirrors 20.
The surfaces of the mirror shields 22 may be treated so that the
heat radiation absorption coefficient of the inner surfaces that
tightly face the mirrors 20 is higher than that of the outer
surfaces that do not tightly face the mirrors 20. By polishing
aluminium, for example, the absorption coefficient can be made as
low as 0.05. A similar effect can be achieved by coating with a
gold layer. By coating with a ceramic layer, such as aluminium
oxide, absorption coefficients of 08. to 0.9 can be achieved. It
should be appreciated that the invention is not limited to these
ways of affecting absorption. Any known technique may be used. The
absorption coefficient of the inner surfaces should be as high as
possible, for example more than 0.8, or even more than 0.9, whereas
the absorption coefficient of the outer surfaces should be as low
as possible, for example less than 0.2, or even less than 0.1. As a
result, a minimum of heat transport is needed for the mirror
shields 22 to prevent heating from other structures.
The outer shield 28 of the support 26 serves to counteract
geometrical deformation of the support 26 due to heating. The outer
shield 28 keeps the temperature of the support 26 at a reduced
level. The temperature of the support 26 approximates a thermal
equilibrium with the outer shield 28, as determined by the heat
transport fluid. Thus geometrical deformation of the projection
system due to deformation of the support 26 is substantially
prevented. In addition, the outer shield 28 counteracts heating of
the support 26 due to radiation from external heat sources.
The outer shield 28 may be shaped and positioned so that it
intercepts a major part of heat radiation to or from the outer
surface of the support 26 and so that at most a minor part (e.g.
<10%) of the intercepted radiation is not to or from the support
26. Any desirable geometrical arrangement of shields may be used
for this purpose.
The surface of the outer shield 28 may be treated so that the heat
radiation absorption coefficient of the inner surface that faces
the support 26 is higher than that of the outer surfaces that do
not tightly face the support 26. The absorption coefficient of the
inner surfaces should be as high as possible, for example more than
0.8, or even more than 0.9, whereas the absorption coefficient of
the outer surfaces should be as low as possible, for example less
than 0.2, or even less than 0.1. As a result, a minimum of heat
transport is needed for the outer shield 28 to prevent heating from
other structures.
The inner shield 29 of the support 26 counteracts geometrical
deformation of the support 26 due to heating by radiation from
exposed surfaces of the mirrors 20 and external heat sources that
are "visible" through holes in the support 26. Large holes have to
be provided to ensure high vacuum. The inner shield 29 keeps the
temperature of the support 26 at a reduced level. The temperature
of the support 26 approximates a thermal equilibrium with the inner
shield 29, as determined by the heat transport fluid. The surface
of the inner shield 29 may be treated so that the heat radiation
absorption coefficient of the inner surface that tightly faces the
support 26 is higher than that of the outer surfaces that do not
tightly face the support 26.
The absorption coefficient of the inner surfaces should be as high
as possible, for example more than 0.8, or even more than 0.9,
whereas the absorption coefficient of the outer surfaces should be
as low as possible, for example less than 0.2, or even less than
0.1. As a result, a minimum of heat transport is needed for the
inner shield 29 to prevent heating from other structures.
Between the support 26 and the mirrors 20 actuators and sensors
(not shown) may be provided to control the position of the mirrors
20. In this case the mirror shields 22 protect the mirrors 20 from
radiation from these actuators and sensors. The inner shields 29
may be placed so as to interpose between the support 26 and the
actuators and sensors to protect the support 26 from heat radiation
from the actuators and sensors.
Although the use of the mirror shields 22, the outer shield 28 and
the inner shield 29 has been described together to achieve a
combined effect, it should be appreciated that each one, or
combination of the shields 22, 28, 29 may be applied also in the
absence of the other ones of these shields 22, 28, 29. For example,
if the main cause of heating is from external heat sources
radiating towards the outer surface of the support 26, the outer
shield 28 may suffice on its own. In this case, the mirror shields
22 may be used in addition when heat problems due to the beam PB
arise. As another example, if the main cause of heating is the beam
PB, the mirror shields 22 on their own may suffice, and the inner
shields 29 may be added to protect the support 26.
Similarly, the absorption coefficients may be varied locally, for
example to provide less absorption at places on the shields 22, 28,
29 that can be cooled less effectively due to space limitations, or
to provide higher absorption at places that are in a reflection
path from a heat source such as a mirror 20 to a temperature
sensitive component such as a mirror 20, or its mirror shield
22.
Although the term "heat transport" has been used throughout for the
fluid and pipes 24, 27, it should be understood that under most
circumstances the pipes 24, 27 serve as cooling pipes and the fluid
serves as cooling fluid, by removing excess heat through the
abduction pipe 18. But without deviating from the invention the
fluid may be used to maintain mirrors 20 and support at a higher
temperature than normally when no beam is present. In this case
heat is normally carried to the mirrors 20, but the amount of
carried heat is reduced when more heat is supplied to the mirrors
20. for example by the beam.
FIG. 3 illustrates a further embodiment in which a control loop is
used to regulate the temperature of mirrors 20 and/or the support
26. In this embodiment, one or more sensors 30 (e.g. a temperature
dependent resistance) are provided on one of more of the mirror
shields 22. A heater 32 is provided coupled to a supply pipe for
the heat transport fluid. A control circuit 34 is coupled between
sensor 30 and the heater 32, arranged to regulate the amount of
heating so that the average sensed temperature is regulated towards
a set temperature. A circulation pump 36 is provided in the supply
pipe. Not shown in the figure is a cooling element, upstream from
heater 32 to remove excess heat from the fluid.
In principle, a common heater 32 may be provided for heating fluid
for all of the mirrors 20 and the support 26. In this case several
sensors may be provided on different heat shields, the average
sensed temperature being regulated. In another embodiment
respective heaters 32 are provided in different parallel parts of
the fluid flow path, so as to adjust the temperature for respective
mirrors or respective groups of mirrors separately, each in
response to a sensor for sensing temperature of the relevant mirror
or mirrors.
However, it should be appreciated that the invention is not limited
to this form of temperature regulation. Temperature regulation,
possibly other than keeping fluid temperature constant, may be
omitted altogether in many cases. Also without deviating from the
invention, sensor 30 may be provided on the mirrors 20, or on the
support 26, or sensors may be provided on both the mirror shield
22, the support 26, and/or the mirror(s) 20. Providing sensors on
the heat shields minimizes mechanical disturbance of the mirrors
20. The fluid may be cooled by a regulated amount instead of
heated. Instead of regulating the amount of cooling or heating,
fluid flow rate may be regulated to regulate temperature.
Although the invention has been described for an embodiment that
uses fluid cooling with fluid circulating through pipes, it should
be appreciated that "fluid" refers to liquid, vapor, gases and
mixtures thereof. It should be appreciated that other forms of
active cooling may be included to transfer heat to or from the heat
shields, such as heat pipes. Furthermore, although a circulating
fluid path was shown, in which cooling fluid is pumped around, it
should be understood that the term "heat transport circuit" as used
herein does not require circulation of fluid. Fresh fluid may be
used instead of circulating fluid.
While specific embodiments of the invention have been described
above, it will be appreciated that the invention may be practiced
otherwise than as described. The description is not intended to
limit the invention.
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